Changes in body composition, including a decrease in bone and muscle mass and an increase in the proportion of fat occur with aging. Lower muscle mass, or the reduction of muscle mass, known as sarcopenia, is associated with lower strength and is thought to contribute to the development of functional limitations and disability in old age. Until the advent of dual-energy x-ray absorptiometry (DXA) scanning, it was difficult to reliably quantify lean mass in large population studies, so the issue of precise clinical definitions had not been paramount. Now that DXA scanning is widely available for osteoporosis screening, the ability to classify large numbers of persons as sarcopenic is feasible. Recent evidence suggests that both lower lean mass and higher body fat may be independently associated with poor function of older adults. Thus, studies are needed to address whether it is possible to intervene in older adults in order to preserve their lean mass and decrease fat mass. Exercise, or increased physical activity, may indeed achieve this goal. This ancillary study will examine whether increased physical activity improves body composition, that is, preserves lean mass and reduces body fat in older adults participating in the LIFE study. LIFE (Lifestyle Interventions and Indpendence For Elders) is a multisite, pilot, randomized controlled trial of the effect of exercise in preventing the loss of ability to walk. The intervention group will engage in aerobic, strengthening, balance and flexibility exercies while the controls will receive a successful aging program that is a series of lectures on topics relevant to the health problems of older people. This study is funded through a cooperative agreement with the Geriatrics and Clinical Gerontology Program at NIA and the DXA research is an add-on substudy. Adding simple measures of body composition to the trial using DXA is an efficient way to address a novel aspect of how increased physical activity may induce favorable changes in body composition in older adults and further, may help determine appropriate interventions to improve body composition and function.